General speaking, the dissolution of precious metals, such as PGM's, was thought to be possible only by the use of aggressive chemicals. A well known process is cyanide dissolution of precious metals.
Dissolution of precious metals with a cyanide solution, e.g., sodium cyanide solution, is relatively easy, particularly the dissolution of PGM's, gold and silver in their elemental state to form cyano-complexes that are stable in aqueous solutions. However, if PGM's exist in mineral form, it may be necessary to liberate the precious-metals, for example, by converting them into their elemental states or other soluble form before dissolving them with a cyanide solution.
A disadvantage of cyanide dissolution is that it presents environmental problems and particular care must be taken in disposing of waste solutions. Moreover, cyanides are costly materials which make their use economically undesirable.
Another aggressive chemical employed to dissolve precious metals is agua regia. Agua regia insures dissolution of PGM's, as well as gold and silver, because of its high acidity, its high redox potential (due to the presence of nitric acid), and its high concentration of complexing ions (12M chloride ions) for complexing the precious metals.
However, the use of aqua regia has its economic disadvantages because of its unacceptable high reagent use, cost and consumption.
Work has been reported regarding the use of bromine for leaching gold from ores. It was found that a relatively low bromide ion concentration can be used to effect dissolution of gold, for example, 0.1M bromide ions, as compared to the much higher chloride ion concentration (12M chloride ions) required for dissolution with aqua regia.
In this connection, reference is made to an article by Batric Pesic et al entitled Dissolution of Gold with Geobrom 3400, Fundamental and Applied Studies, which was delivered at the Proceedings of the 13th IMPI Conference at Montreal Precious Metals 1989.
Geobrom 3400 (which is the trademark of a product of Great Lake Chemical Corporation) is a solution containing about 34% free bromine. According to the aforementioned article, the bromine solution was used to dissolve gold from gold concentrates and electronic scrap. Rotating disc studies were conducted using a rotator a speed controller, a reactor, and a water bath. A typical experiment comprised a 500 ml solution containing 5 ml/l of Geobrom 3400 at a natural pH at a temperature of 25.degree. C., the stirring being carried out at 500 rpm.
The studies indicated that bromine provided a substantially higher rate of gold dissolution than dissolution with sodium cyanide or thiourea.
The use of bromine for the dissolution of gold is disclosed in another article entitled Leaching and Recovery of Gold From Black Sand Concentrate and Electrochemical Regeneration of Bromine by A. Dadgar et al. This article was presented before the Society for Mining, Metallurgy, and Exploration, Inc. at Reno, Nev. on Sep. 10-12, 1990. This paper describes the leaching of gold with Geobrom 3400 from very rich black sand concentrate and its subsequent recovery by ion exchange resins and solvent extraction.
We have discovered a method for the dissolution of precious metals, e.g., PGM's, gold and silver by simply using a sulfuric acid solution relatively low in halide concentration and of controlled redox potential at least sufficient to convert the precious metal to an ionic form conducive to forming a soluble bromide complex thereof.